Customer Credits product and affiliate grants

Creem Customer Credits currently work as a ledger-style API, but common credit flows still require app-side fulfillment logic.

Creem should let merchants configure credit grants directly on products, including credit account/name, amount, unit label, whether credits are granted once or on every recurring renewal, and refund/reversal behavior.

Creem should also support Customer Credits rewards for affiliate or referral attribution.

This would make credit packs, subscription credit allowances, renewals, affiliate rewards, and refund handling first-class Creem billing flows instead of custom app-side mappings.